An earth drill, which excavates material by taking in excavated soil into a bucket while excavating the bottom of a bore hole with a cutter provided at a conical bottom cover, is normally ideal for an excavating operation on a soil stratum that can be represented with an N value. However, it is not suited for so-called barrier excavation including excavation of stones such as cobblestones and boulders that are too large to fit into the drilling bucket and excavation of a concrete slab. If the cutter strikes such a layer during an excavating operation, special measures must be taken by, for instance, removing the cobblestones with a hammer grab bucket mounted at a separate crane or using an all casing machine.
The inventor of the present invention et al. proposed in Japanese Unexamined Patent Publication No. H 11-141261 a means for solving the problem described above, achieved by mounting a detachable cylindrical bucket at the kelly bar instead of the drilling bucket for the earth drill having the bottom cover and by also providing a auger type excavating tool that forms a small hole in the vicinity of its circular drilling groove to allow excavated material to be drawn out through the hole.
However, there is a problem with an earth drill that utilizes a cylindrical bucket as disclosed in Japanese Unexamined Patent Publication No. H 11-141261 in that a separate lifting means is required to lift the excavated round slab, rocks and the like.
Accordingly, the inventor of the present invention et al. proposed an excavating tool for an earth drill in Japanese Unexamined Patent Publication No. 2001-90465, which allows excavated material to be lifted to the ground level during a barrier excavating operation of slab, cobblestones, boulders or the like executed with the earth drill.
The excavating tool disclosed in the publication described above comprises an inner member having an upper end thereof connected to the kelly bar, an outer member and a second member provided between the inner member and the outer member. These members are combined so as to allow them to move up/down relative to one another. In addition, a cylindrical bracket having an cutting teeth at the lower end thereof and an open bottom is mounted at the outer member. At the outer member, a grab bucket housed inside the cylindrical bucket and constituted of a pair of jaws which grab objects excavated with the cylindrical bucket is provided as well.
The lower end of the second member is linked to the jaws at the grab bucket so as to open and close the grab bucket. The excavating tool adopts a structure that closes the grab bucket as the second member moves upward relative to the outer member and opens the grab bucket as the second member moves downward relative to the outer member. Namely, the inner member and the second member are connected with each other through a first hydraulic cylinder and second hydraulic cylinders provided between the second member and the grab bucket. Pressure is generated as the kelly bar is lifted and the first hydraulic cylinder consequently extends, which communicates the pressure to the second hydraulic cylinders to extend the second hydraulic cylinder. This lifts the second member relative to the outer member, and thus, the grab bucket mounted at the outer member closes to trap soil.
The excavating tool adopts a structure that automatically releases the binds among the inner member, the second member and the outer member achieved with cotters to allow them to extend as the members held in a most contained state achieved with the cotters are lowered to the bottom of the bore hole. Then, as the kelly bar is rotated, the cylindrical bucket and the grab bucket are caused to rotate together with the inner member, the second member and the outer member to cut into the earth. When the kelly bar is lifted after the earth is cut to a predetermined depth, the inner member is lifted together with the kelly bar while the outer member remains at the bottom of the bore hole due to the dead weight of the cylindrical bucket, the grab bucket and the like. As a result, the grab bucket, with its jaws linked to the second member, closes through the operation described above to take in the soil to be excavated. After the grab bucket is closed, the outer member is lifted to the ground level together with the cylindrical bucket and the grab bucket. As the cylindrical bucket is set on the ground and the kelly bar is lowered, the first hydraulic cylinder contracts, the pressure generated in the pressure oil through the contraction of the first hydraulic cylinder causes the second hydraulic cylinders to contract as well and the second member is lowered as a result, thereby allowing the grab bucket to open to discharge the excavated soil onto the ground.